1. Unit 2: Industrial Instrumentation
Topic: FLOW MEASUREMENT
BY: MOHSIN MULLA
ASST. PROFESSOR, DEPT. EE
2. • Accurate measurement of flow rate of liquids and gases is an essential
requirement for maintaining the quality of industrial processes. In
fact, most of the industrial control loops control the flow rates of
incoming liquids or gases in order to achieve the control objective. As
a result, accurate measurement of flow rate is very important.
Introduction
3. The common types of flowmeters
• (1) Obstruction type (differential pressure or variable area)
• (2) Inferential (turbine type),
• (3) Electromagnetic,
• (4) ultrasonic and
• (5) Anemometer,
• (6) Mass flowmeter (Coriolis).
4. Obstruction type flowmeter
• Obstruction or head type flowmeters are of two types: differential
pressure type and variable area type.
• Orifice meter, Venturimeter, Pitot tube fall under the first category,
while.
• rotameter is of the second category.
• In all the cases, an obstruction is created in the flow passage and the
• pressure drop across the obstruction is related with the flow rate.
5. Theory of Fixed Restriction Variable Head Type
Flowmeters
• In the variable head type flow meters, a restriction of known dimensions
is generally introduced into pipeline, consequently there occurs a head
loss or pressure drop at the restriction with increase in the flow velocity.
Measurement of this pressure drop is an indication of the flow rate.
6. Orifice Flow Meter
• The orifice plate inserted in the
pipeline causes an increase in
flow velocity and a
corresponding decrease in
pressure. The flow pattern
shows an effective decrease in
cross section beyond the orifice
plate, with a maximum velocity
and minimum pressure at the
venacontracta.
9. Venturi Tubes
• Venturi tubes are differential pressure producers, based on Bernoulli’s
Theorem. General performance and calculations are similar to those for
orifice plates.
10. Construction of Venturi tube
• It consists of a cylindrical inlet section equal to the pipe diameter ; a
converging conical section in which the cross sectional area decreases
causing the velocity to increase with a corresponding increase in the
velocity head and a decrease in the pressure head ; a cylindrical
throat section where the velocity is constant so that the decreased
pressure head can be measured ; and a diverging recovery cone
where the velocity decreases and almost all of the original pressure
head is recovered. The unrecovered pressure head is commonly
called as head loss.
13. • When a solid body is held centrally and stationary in a pipeline with a fluid streaming
down, due to the presence of the body, the fluid while approaching the object starts losing
its velocity till directly in front of the body, where the velocity is zero. This point is known
as the stagnation point. As the kinetic head is lost by the fluid, it gains a static head. By
measuring the difference of pressure between that at normal flow line and that at the
stagnation point, the velocity is found out. This principle is used in pitot tube sensors.
• The simplest pitot tube consists of a tube with an impact opening of 3.125 mm to 6.35 mm
diameter pointing towards the approaching fluid. This measures the stagnation pressure. An
ordinary upstream tap can be used for measuring the line pressure.
19. Working:
• The turbine flow meter is mainly used for the purpose of measurement
of liquid and gas at very low flow rates. A simple turbine flow meter
shown in Fig. provides a frequency output signal that varies linearly
with volumetric flow rate over specified flow ranges. The entire fluid
to be measured enters the flow meter, then passes through a rotor. The
fluid passing through the rotor causes it to turn with an angular
velocity that is proportional to the fluid linear velocity. Therefore, the
volumetric flow rate is linear within the given limits of flow rate.
20. Electromagnetic Flow Meter
• Electromagnetic flow meters use Faraday’s law of electromagnetic
induction for making a flow measurement. Faraday’s law states that,
whenever a conductor of length ‘l’ moves with a velocity ‘v’
perpendicular to a magnetic field ‘B’, an emf ‘e’ is induced in a
mutually perpendicular direction which is given by
e = Blv
• where
B = Magnetic flux density (Wb/m2)
l = length of conductor (m)
v = Velocity of the conductor (m/s)
21.
22.
23. • In the electromagnetic meter, the constant magnetic field is generated
around the pipe by magnet and the flowing liquid acts as a conductor.
The flowing liquid can be regarded as a continuous series of discs
passing through the magnetic field, the bore of the pipe being directly
proportional to length of the conductor. Thus the emf generated is
directly proportional to the velocity of flow.
24.
25. Advantages of Electromagnetic Flow Meter
• (i) The obstruction to the flow is almost nil and therefore this type of meters can be used for
measuring heavy suspensions, including mud, sewage and wood pulp.
• (ii) There is no pressure head loss in this type of flow meter other than that of the length of
straight pipe which the meter occupies.
• (iii) They are not very much affected by upstream flow disturbances.
• (iv) They are practically unaffected by variation in density, viscosity, pressure and temperature.
• (v) Electric power requirements can be low (15 or 20 W), particularly with pulsed DCtypes.
• (vi) These meters can be used as bidirectional meters.
• (vii) The meters are suitable for most acids, bases, water and aqueous solutions because the
lining materials selected are not only good electrical insulators but also are corrosionresistant.
• (viii) They are capable of handling extremely low flows.
27. These device measure flow by measuring the time taken for an ultrasonic energy
pulse to traverse a pipe section, both with and against the flow of the liquid within
the pipe.
28.
29. Anemometers
• The use of hot wire resistance transducer is to measure the flow rate of
fluids by means of measuring velocity of non conducting liquids. In open
channels and closed pipes it can be conveniently measured by suitably
locating the hot wire filament.
HOT wire Anemometer
30. Principle
• In hot wire anemometers, the filament is heated initially by means of
passing an electric current. This heated resistive filament mounted on a
tube is exposed to air flow or wind, which is cooled because of
fanning effect. Depending on the velocity of air flow, the amount of
cooling varies. The resistance of the probe when it is hot is different
from that when it is cooled. This difference in resistance or this
variation in resistance is converted into a voltage variation and thereby
the flow velocity is converted into a voltage variation.
33. Criollis Mass flow meter
• The basic operation of Coriolis flow meters is based on the principles
of motion mechanics. As fluid moves through a vibrating tube it is
forced to accelerate as it moves toward the point of peak-amplitude
vibration. Conversely, decelerating fluid moves away from the point of
peak amplitude as it exits the tube. The result is a twisting reaction of
the flow tube during flowing conditions as it traverses each vibration
cycle.
Editor's Notes
Working The flowing fluid impinges on the blades of turbine (rotor), imparting a force to the blade surface which causes the rotation of the rotor. At a steady rotational speed, the speed of the rotor is directly proportional to the fluid velocity, and hence to volumetric flow rate. The speed of rotation is monitored in most of the meters by a magnetic pick-up coil, which is fitted to the outside of the meter housing. The magnetic pick-up coil consists of a permanent magnet with coil windings which is mounted in close proximity to the rotor but external to the fluid channel. As each rotor blade passes the magnetic pick-up coil, it generates a voltage pulse which is a measure of the flow rate, and the total number of pulses give a measure of the total flow. By digital techniques, the electrical voltage pulses can be totalled, differenced and manipulated so that a zero error characteristic of digital handling is provided from the electrical pulse generator to the fluid readout.
